Color sensor

ABSTRACT

A hand-manipulatable device includes a sensor for gathering reflective, densitometric, spectrophotometric, colorimetric, self-luminous or radiometric readings from a sample surface. The device includes a housing having a substantially flat bottom surface and a top surface contoured to fit comfortably in the fingers and palm of the human hand. The housing also includes area on its top surface for seating an index finger of the human hand. Positioned within this area is a pressure-activated switch that is operatively coupled to the sensor circuitry for performing the readings. Preferably, the sensor is mounted into the device such that the focal aperture of the sensor is in axial alignment with the pressure-activated switch. Accordingly, a user will be able to use the device to &#34;point&#34; with his or her index finger to an area of the sample surface, and will then simply press the switch to initiate the readings, using the same index finger.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 from ProvisionalApplication Ser. No. 60/025,911, filed Sep. 12, 1996.

BACKGROUND

The present invention relates generally to devices for measuringreflective, transmissive, or self-luminous samples and reporting theirspectrophotometric, spectroradiometric, densitometric, or othercolorimetric appearance attributes.

Users of desktop color systems have a need to accurately measure color.Color systems end-users expect accurate color matching between theirsource (scanner or monitor) and the color hardcopy produced by theircolor digital printer. In order to achieve WYSIWYG ("What You See IsWhat You Get") color, color imaging devices must be characterized andcalibrated transparently to the user, and work seamlessly with popularsoftware applications. Support for import and export of image datadescribed in a device independent format such as CIE is increasingly arequirement.

The inputting of colors represented by physical samples into anelectronic design for display and printing is currently a tediousprocess. The end-user has several options. The sample may be scanned,but the color reproduction will be poor for the reasons discussed above.The sample may be visually matched by specifying RGB or CMYK amounts forthe application, but the match will be device dependent and highlyvariable. A color specification system may be employed by visuallymatching the sample to one of the specified colors then entering thecolor specification to the application, but the match will be devicedependent, highly variable, and rely upon application compatibility withthe color specification system.

One application where WYSIWYG is particularly important is color desktoppublishing. In many desktop and workstation environments, colorend-users (e.g. publishing, prepress, design, graphics, etc.) have awide selection of input and output devices (e.g. scanners, monitors,printers, imagesetters, etc.) and color creation applications. Sincethis environment is generally open-architecture, the colorimetriccharacteristics of the various devices and applications are not, andcannot be, well-matched due to the multi-vendor nature of the market. Asa result, the quality of color reproduction between input and output ishighly variable and generally poor. In order to achieve consistent colormatching in this highly disparate environment, several companies haveintroduced software-based color management system (CMS) technology basedon device independent color. The principle of operation of such systemsis to reference all devices (device dependent) to a common CIE colorspace (device independent). A simple work flow for such a CMS might becomprised of the following steps: (1) the image scan (Scanner RGB) isreferenced to CIE; (2) the image is converted to Display RGB forediting; (3) the displayed image is referenced back to CIE; and (4) theimage is converted to printer CMYK for output.

Generally, the CMS vendor provides several key elements: a library ofdevice characterization profiles, a color matching method, and colortransformation software. Since the expertise and equipment required forthe creation of profiles is expensive, device characterization isperformed by the CMS vendor for an average device for average viewingconditions. Unfortunately, characterization accomplished in this manneris done under factory conditions and not for the end-user conditions(device, light source, media, viewing, etc.). Since an end-user's devicewill be quite different from the device that was profiled at thefactory, some vendors offer hardware/software calibration. Calibrationonly partially compensates for the differences between factoryconditions and end-user conditions but does not satisfy the need forend-user device characterization compatible with open architecture CMS.However, there are no end-user characterizors for ambient, display, orhardcopy characterization due to the high cost of such equipment andlack of suitable end-user software. Currently, CMS products aregenerally proprietary and must be purchased from the CMS vendor. As openarchitecture CMS technology will be included as part of the operatingsystem, device manufacturers, third parties, and end-users will requirecustom CMMs or profiles.

The combination of CMS technology, factory characterization, andend-user calibration improves the average color reproduction on thedesktop somewhat, but is expensive, complex, and does not meet theend-user requirements for quality, cost, speed, and compatibility.End-users need a fast, low-cost, simple hardware/software characterizorenabling them to easily characterize (not just calibrate) their specificscanners, monitors, and printers to their specific viewing conditions ina manner that is compatible with open architecture CMS technology.

U.S. Pat. No. 5,137,364 discloses a color sensor that employs aplurality of LEDs (light emitting diodes) and an array ofphotodetectors. The McCarthy sensor utilizes individually addressable,customized LEDs as its illumination sources and a photodetector array,where each photodetector measures reflectance of the sample at adifferent visible wavelength, and where each element's spectralsensitivity must be individually optimized. The McCarthy sensor alsoutilizes a single beam reflectance measurement scheme. Thus, thephotodetectors must be made as stable as possible by maintaining at aconstant temperature and being protected from humidity, etc.Furthermore, the McCarthy device requires expensive and stablecomponents since the measured energy collected from the color samplemust be constant. Such expensive and customized components neverthelessmay also experience long term drift and may be highly sensitive tonoise.

U.S. Pat. No. 5,377,000 to Berends discloses a color sensor thatutilizes a single illumination source positioned directly above thesample, 21 sample photodetectors arranged circumferentially around theillumination source and positioned at 45° angles with respect to thesample, and 2 reference photodetectors positioned to receive lightdirected from the single illumination source. Berends utilizes a"pseudo-dual beam" reflectance measurement scheme in an attempt toeliminate the need of an additional 19 reference photodetectors thatwould be required in a classic dual-beam reflectance measurement scheme.This is performed by configuring the 2 reference photodetectors tosample the illumination source at opposite extremes of the visible lightspectrum, and by applying a "least squares fit" calibration calculationto simulate the required 21 reference channel readings.

SUMMARY

The present invention provides an extremely accurate color sensor,designed to utilize a classic dual-beam reflectance measurement schemewith inexpensive and commercially available components.

In accordance with one aspect of the present invention, a device formeasuring reflective, transmissive, or self-luminous samples, comprises:a plurality of light sources, which are preferably light emitting diodes(LEDs), where each of the light sources emit light of a substantiallydifferent wavelength band spaced in the visible spectrum; a referencechannel photodetector; a sample channel photodetector; an optical capadapted to direct a first portion of the light emitted by each of thelight sources to the reference channel photodetector; a reflector conefor directing a second portion of the light emitted by each of the lightsources to the sample; and a receptor piece for directing the diffuseportion of the light reflected from the sample to the sample channelphotodetector.

In a preferred embodiment of the invention, the reference channel andsample channel photodetectors are identical devices and are mountedback-to-back to share environmental characteristics, and in turn,minimize the variation between their respective responses. The outputsignals from the photodetectors are processed to provide at least oneappearance value, e.g., spectrophotometric, spectroradiometric,densitometric, and other colorimetric appearance attributes for thesamples.

Preferably, the optical cap has a non-absorbing interior integratingsurface and is mounted over the LEDs and the reference photodetectorsuch that the reference portion of light generated by the emittersreflects off of the integrating surface to the reference photodetector.The LEDs are each preferably mounted such that a portion of each LEDextends into a cylindrical cavity pointing downwards towards the sample,the cavities acting to collimate the sample portion of light emitted bythe LEDs. The reflector cone preferably has a conical reflectingsurface, positioned in alignment with the cavities, and angled 22.5°with respect to the cavities' axes, such that the sample portion oflight directed downward towards the conical reflecting surface willfirst reflect off of the conical reflecting surface and then be directedtowards the sample at an angle of 45°. Therefore, the samplephotodetector, positioned directly over the sample and behind anaperture in the receptor piece, will receive the diffuse component ofthe light reflected from the sample.

The device may further comprise means for supplying current to the LEDwherein the LED current provided by the current supplying means isprogrammed such that assumed incident light which is proportional to theLED current is a known measurement.

The invention also provides a method of measuring color samples using acolor measurement device, the method comprising: (a) activating at leastone light source of a certain wavelength band, (b) directing a firstportion of the light emitted by the light source to a referencephotodetector, (c) directing a second portion of the light emitted bythe light source to the sample surface, (d) directing light reflectedfrom the sample surface to a sample photodetector, (e) calculating areflectance for the light source based upon output readings of thesample photodetector in step (d) and output readings of the referencephotodetector in step (b), and (f) repeating steps (a) through (e) forseveral light sources, each emitting light of substantially differentwavelength bands.

The method, in a preferred embodiment, also includes the steps of (f)obtaining a reading from the sample photodetector when none of the lightsources are illuminated and (g) obtaining a reading from the referencephotodetector when none of the light sources are illuminated, where step(e) involves the step of calculating a reflectance for the light sourcebased upon a ratio of the difference of output readings of the samplephotodetector in step (d) and output readings of the samplephotodetector in step (f) versus a difference of output readings of thereference photodetector in step (b) and output readings of the referencephotodetector in step (g). And the method, in the preferred embodimentfurther includes the steps of (h) activating the light source of step(a), (i) directing a first portion of light emitted by the light sourceto a reference photodetector, (j) directing a second portion of thelight emitted by the light source to a substantially non-reflectingcalibration surface, (k) calculating a "black" calibration reflectancereading for the light source based upon the sample photodetector readingfrom step (j) and the reference photodetector reading from step (i), (l)again activating the light source of step (a), (m) directing a firstportion of light emitted from the light source to the referencephotodetector, (n) directing a second portion of light emitted from thelight source to a substantially white calibration surface, (o)calculating a "white" calibration reflectance reading for the lightsource based upon the sample photodetector reading from step (n) and thereference photodetector reading from step (m), and (p) calculating anormalized and bias corrected reflectance for the light source using thereflectance calculated in step (e), the "black" calibration reflectancecalculated in step (k) and the "white" calibration reflectancecalculated in step (o).

In another embodiment, step (e) includes the step of accounting for thesensitivity of the reference photodetector and the spectral powerdistribution of the light source. In yet another embodiment, the methodfurther comprises the step of profiling the characteristics of thereference photodetector and the light source by performing steps (a)through (f) on at least two samples having known reflectances. And inyet another embodiment, the method further comprises the step oftransforming the reflectance calculated in step (e) into a CIE XYZtristimulus value for the sample, where the transformation accounts forthe sensitivity of the human visual system.

Accordingly, a preferred embodiment of the present invention provides acolor sensor which utilizes the "dual beam" reflectance sensing schemeby incorporating a reference photodetector. Because both the samplephotodetector and the reference photodetector share common optical,electrical, environmental and mechanical characteristics, the presentinvention can utilize low price components which may tend to exhibitdrift. Further, because the sample and reference channels are ratioed incalculating the reflectance, this drift will be canceled out, therebyyielding ultra-high performance at very low cost. The present inventionhas also been designed to, and physically configured to, allow themeasurement of reflected light from an area of 3 millimeters in diameterusing illumination with 45° incidence and 0° detection angle. The sensoris highly rugged, having no moving parts, and through the application ofthe dual optical paths, is extremely reliable and inexpensive.Off-the-shelf photodetectors with integrated amplifiers reduce darkerrors, noise, cost and allow for accurate gain tracking over theiroperational temperature range. The design also accommodates aninterchangeable selection of LEDs, allowing the latest, most efficientand least costly commercially available LEDs to be used.

The present invention is also designed to be easily assembled and can beintegrated into a main circuit board to further reduce assembly costs.Furthermore, optical and beam-directing components of the presentinvention can also be integrated directly as part of an outer hand-heldenclosure (i.e., hand-held mouse), further increasing the systemintegration, and therefore, further reducing the end user price. Becausetemperature stabilization and compensation is no longer required, LEDreliability is enhanced. Since the heat producing LEDs are now isolatedfrom the thermally sensitive photodiodes, there is also a reducedopportunity for adverse thermal shock. Furthermore, the presentinvention reduces the consumed power and reduces measurement overhead.Thus, the invention is specially designed to be inexpensivelymass-produced, without sacrificing accuracy or reliability.

As indicated above, one aspect of the present invention incorporates thecolor sensor into a hand-held "mouse" device. The mouse device includesan area on its top surface for seating an index finger of the humanhand. Positioned within this area is a pressure-activated switch that isoperatively coupled to the circuitry for performing the colorimetric andreflectance readings. Preferably, the sensor is mounted into the mousedevice such that the focal aperture of the downward pointing reflectorcone is in axial alignment with the pressure-activated switch.Accordingly, a user will be able to use the mouse to "point" with his orher index finger to an area of the sample surface, and will then simplypress the switch using the same index finger.

Other foreseeable applications for the color sensor of the presentinvention include high-speed color-inspection/control in a manufacturingor production environment, analogous to the uses described in U.S. Pat.No. 5,021,645 to Satula et al.; or color matching of cosmetics orclothing accessories to a skin-color or foundation make-up color of acustomer, analogous to the uses described in U.S. Pat. No. 5,537,211 toDial.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional, elevational view of an embodiment of thecolor sensor of the present invention;

FIG. 2 is a top view of the annular collar component of the colorsensor;

FIG. 3 is a side view of the annular collar component of the colorsensor;

FIG. 4 is a top view of the color sensor of an embodiment of the presentinvention as incorporated into a circuit board;

FIG. 5 is a cross-sectional, elevational view of the embodiment of FIG.4;

FIG. 6 is a cross-sectional, elevational, and exploded view of anembodiment of the color sensor and circuit board of the presentinvention;

FIG. 7 is a schematic block diagram representation of circuitry for usewith the present invention;

FIG. 8 is a schematic block diagram representation of alternatecircuitry for use with the present invention;

FIG. 9 is a perspective view of a hand-held "mouse" device incorporatingthe present invention;

FIG. 10 is a top view of the hand-held "mouse" device of FIG. 9;

FIG. 11 is a cross-sectional, elevational view of the hand-held "mouse"device incorporating the present invention;

FIGS. 12a-c respectively show a top view, one cross-sectionalelevational view, and another cross-sectional elevational view of analternate embodiment of the annular collar component of the presentinvention;

FIG. 13 is a cross-sectional, elevational view of an alternateembodiment of the color sensor of the present invention;

FIG. 14 is a cross-sectional, elevational view of another alternateembodiment of the present invention;

FIG. 15 is a cross-sectional, elevational view of another alternateembodiment of the present invention;

FIG. 16 is a cross-sectional, elevational view of another alternateembodiment of the collar and reflector cone components of the presentinvention; and

FIG. 17 is a cross-sectional, elevational view of another alternateembodiment of the collar and reflector cone components of the presentinvention.

DETAILED DESCRIPTION

As shown in FIG. 1, the color sensor 10 for sampling the color of asample surface 12 comprises a printed circuit board 13; a plurality oflight sources, such as light emitting diodes (LEDs) 18 mounted inapertures 19 extending through the circuit board 13, each of the LEDsemitting light of a substantially different wavelength band spaced inthe visible spectrum; a reference photodetector 20 mounted to the topsurface 14 of the printed circuit board; a sample photodetector 22mounted to the bottom surface 15 of the printed circuit board,substantially back-to-back with the reference photodetector 20; anannular collar 24 mounted to the bottom surface 15 of the printedcircuit board; an optical cap 26 mounted to the top surface 14 of theprinted circuit board; and a reflector cone 28 mounted onto the collar24.

As shown in FIGS. 1, 2, and 3, the collar 24 is an annular componenthaving a plurality of emitter apertures 30 bored through the collar inan axial direction, and circumferentially spaced around a receiveraperture 32 that is axially bored through a frustoconical receptor piece36. The receptor piece 36 extends and points downward from the bottomsurface 38 of the collar 24 along axis A. A rectangular cavity 40 forhousing the sample photodetector 22 extends into the collar 24 from thetop surface 42 of the collar, substantially at axis A, and is in directoptical communication with the receiver aperture 32, such that lightwaves reflected into the receiver aperture 32 of the receptor piece 36can contact the sample photodetector 22.

Those skilled in the art will appreciate that the same cavity 40 forhousing the sample photodetector 22 can also be used to mount anoptional optical filter 41 constructed of flat glass or plastic. Such afilter 41, mounted at the bottom of the cavity 40, at the opening of thereceiver aperture 32, and aligned with axis A, could serve the purposeof excluding any unwanted light, e.g., infrared light in the case of asensor optimized for the visible spectrum. Accordingly, the filter 41could be an infrared filter or any other band-pass filter asnecessitated by the envisioned use of the sensor 10. The filter 41 willalso protect the face of the sample photodetector. It is also within thescope of the invention to provide a similar optical filter over thereference photodetector 20 to band-pass filter the light reflectedthereto.

As shown in FIG. 1, the plurality of LEDs 18 are preferably mounted intoapertures 19 of the printed circuit board 13 such that a light emittingportion of each LED extends into a corresponding one of the emitterapertures 30. The optical cap is mounted to the printed circuit board 13such that the cap forms an enclosed cavity 44 over the entire array ofLEDs 18. Each of the plurality of LEDs are also mounted on the printedcircuit board 13 such that a portion of the light emitted by the LED isemitted or reflected backwards (upwardly) into the cavity 44 formed bythe optical cap 26. The reference photodetector 20 is centrally mountedwith respect to the array of LEDs 18 to the top surface 14 of theprinted circuit board, within the cavity 44. The inner surface 46 of theoptical cap is preferably coated with an opaque, substantiallynon-absorbing, integrating coating such as a flat white paint.Therefore, light waves emitted from the LEDs through the top surface 14of the printed circuit board, and into the cavity 44, are transmitted tothe integrating surface 46 and are directly or indirectly reflected fromthe integrating surface to the reference photodetector 20.

The reflector cone 28 is attached to the bottom 38 of the collar and hasan aperture 48 through the tip 50 of the reflector cone. The aperture 48is in axial alignment with the axis A of the collar 24, and in turn, isin alignment with the receiver aperture 32 of the collar. The reflectorcone 28 has a substantially frustoconical inner surface which forms acavity 53 between the collar 24 and the reflector cone 28. The reflectorcone 28 also has an conical reflector surface 52, preferably coated witha chrome plating (or any other suitable reflective coating), inalignment with each of the axes B of the emitter apertures 30, and whichis angled, with respect to the axes B, inwardly towards the axis A, atan angle that is approximately 22.5°. Therefore, the light emittedthrough the emitter apertures 30 and reflected from the conicalreflective surface 52 will contact the aperture 48 of the reflectorcone, and in turn the sample surface 12, substantially at an angle of45° with respect to the flat tip 50 of the reflector cone (or withrespect to the sample surface 12, if applied to a sample). Thus, thediffuse component of the reflected light waves will be transmittedupwardly from the sample along axis A, through the receiver aperture 32to the sample photodetector 22.

Accordingly, when one of the LEDs is activated, and when the flat tip 50of the reflector cone is abutting a sample surface 12, the referencephotodetector 20 will sample light waves emitted from the portion of theLED 18 extending through the top surface 14 of the printed circuit boardand reflected from the optical cap's inner surface 46, and the samplephotodetector 22 will sample the diffuse component of the light wavesemitted from the portion of the same LED 18 extending through the bottomsurface 15 of the printed circuit board and reflected off the samplesurface 12. The use of the 45°/0° geometry of the present inventioncorresponds more closely with the visual viewing of samples and excludesthe specular component of reflectance. The spectral reflectance of thecolor sample can thus be calculated for this particular LED (orcombination of LEDs) from the readings of the reference photodetector 20and the sample photodetector 22 using a dual beam method as is describedin detail below.

When using the 45°/0° geometry, the operable range for the angle of theconical reflector surface is 20.5° to 24.5°, while the preferred rangeis 22.4° to 22.6°. Thus, those skilled in the art will appreciate thatit is within the scope of the present invention that the angle of theconical reflector surface be within the above ranges for the 45°/0°geometry. It will also be appreciated to those of ordinary skill in theart that angle A can be altered to obtain optical geometries that arebetter suited for particular uses. For example, when using the presentinvention to detect characteristics of a patient's skin (such as skincolor), a 20°/0° geometry as shown in FIG. 16 or modified-diffuse/0°geometry as shown in FIG. 17 may be preferred. These alternateembodiments are better suited for remote color sensing of a samplesurface and will be discussed in greater detail below.

Referring again to the embodiment shown in FIG. 1, each emitter aperture30 is preferably formed with an upper cylindrical channel 54 and a lowercylindrical channel 56. The lower cylindrical channel is concentricallyaligned with the upper cylindrical channel 54 along the axis B of theemitter aperture and has a smaller diameter than the upper cylindricalchannel 54. The lower cylindrical channel thus acts to collimate thelight waves emitted through the emitter aperture 30, to the conicalreflective surface 52, and onto the sample surface 12. Additionally,although shown as having the same size, the emitter apertures 30 canvary in size to allow for maximum flexibility in LED selection.

As shown in FIG. 6, in an alternate embodiment of the invention, and tosimplify mass-production of the device, the LEDs 18' can be mounted to aseparate daughterboard 162 that, in turn, is mounted to the top surface14' of the main circuit board 13' via mating connectors 164, 166. Themain circuit board 13' will contain the electronics (described below)that drive the LEDs 18', and the electronic signals for driving theLEDs, produced by the main circuit board 13', will be passed to thedaughterboard through the mating connectors 164, 166. When thedaughterboard is mated to the main circuit board, the LEDs 18' willextend into the apertures 19' extending through the main circuit board13'. An optical cap 26' designed to be fitted over the daughterboard 162will be mounted to the top surface 14' of the main circuit board 13' andwill act to reflect light emitted from the LEDs 18' to the referencephotodetector 20 as described above. Likewise, the collar 24 andreflector cone 28 will be mounted to the bottom surface 15' of the maincircuit board 13' and will act to direct light emitted from the LEDs 18'to the sample surface at approximately 45°, and to direct the diffusecomponent of the light reflected from the sample to the samplephotodetector 22 as described above.

Those skilled in the art will appreciate that the light sources can bemounted or positioned in other ways; so long as a portion of the lightemitted from the light source is transmitted or reflected through theemitter aperture 30 and another portion of the light emitted from thelight source is emitted or reflected into the cavity 44 within theoptical cap 26. For example, it is within the scope of the invention tomount each LED to the bottom surface 15 of the printed circuit board,extending completely within the emitter aperture 30, and to bore a holethrough the printed circuit board 13 above the LED 18 such that thelight emitted from the LED 18 will be transmitted through the hole andinto the cavity 44 and will also be transmitted through the lowercylindrical channel 56 and into the cavity 53 as described above. Inanother example, as shown in FIG. 15, the LEDs can be clustered at anylight-shielded point of circuit board; and when activated, an acrylic"light-pipe" 180 can be used similar to a fiber optic element, to directa first fraction 182 of light waves to the sample surface 12 so that itis incident at 45°. Of course, the sample photodetector 22 will be inposition to detect the diffuse component of the light reflected from thesample surface. Because the light-pipe 180 tends to bleed light wavesradially therefrom, the reference photodetector 20 can be in anyshielded position to receive a fraction 184 of light bleeding from thelight-pipe 180.

It will also be apparent to those skilled in the art, thatphotodetectors for use with present invention can includephotoconductive cells, photodiodes, photoresistors, photoswitches,phototransistors, phototubes, photovoltaic cells, light-to-frequencyconverters, or any other type of photosensor capable of converting lightinto an electrical signal. Such photodetectors can include integratedconversion of light to voltage with electronic amplification components;integrated conversion of light to digital frequency components; orintegrated analog to digital conversion components.

As shown in FIGS. 4 and 5, the color sensor 10 is preferably mounted tothe distal or pointed end 59 of a pointed printed circuit board 13. Thecircuitry for controlling the LEDs and photodetectors, and fortransferring the measured signals to a host computer are also mounted tothe printed circuit board 13. The reference photodetector 20 and thesample photodetector 22 are shown in FIG. 5 as off-the-shelf chipdevices mounted to the printed circuit board 13 via pins 60. The LEDsare also off-the-shelf devices mounted to the printed circuit board 13via leads 61. Suitable photodetectors for use with the present inventionare OPT209PJ from Burr-Brown or TSL230A from Texas Instruments; andsuitable LEDs for use with the present invention are NLPB-300A fromNichia, NSPB-300A from Nichia, NSPG-300A from Nichia, E166 from Gilway,E104 from Gilway, E198 from Gilway, E102 from Gilway, E472 from Gilway,BL-B4331E from America Bright, or HLMP-K640 from Hewlett Packard.Preferably, the LEDs are encapsulated so as to provide a lens integralwith the LED. Optionally, a non-encapsulated light source can beutilized by incorporating a lens within the emitter aperture 30.

As shown in FIGS. 9-11, a shell 62, consisting of an upper shell piece64 and a lower shell piece 66 is mounted around the printed circuitboard 13 and color sensor 10 and is ergonomically shaped for grippingand manipulation by an operator's hand. The shell contains apressure-activated switch 68 that is operatively coupled to thecircuitry contained on the printed circuit board, such that the operatorwill be able to initiate a color measurement process by activating theswitch. Preferably, the optical cap 26 is an integral part of the uppershell piece 64 and the collar 24 is an integral part of the lower shellpiece 66. The reflector cone 28 is preferably an independent piece whichis mounted to the shell 62 after mounting the upper shell piece 64 andlower shell piece 66 to the printed circuit board 13. The separatesensor parts, the upper shell piece 64, the lower shell piece 66 and thereflector cone 28 are all preferably made from an opaque black ABSplastic.

As shown in FIGS. 10 and 11, upper shell piece 64 has an area 168 forseating an index finger 170 of a human hand. Positioned in this area 168is the switch 68, operatively coupled to the circuitry of the sensorsuch that pressure activation of the switch will activate the circuitry,and will thus initiate a colorimetric reading of the sample surface 12.The switch 68 is in axial alignment with the aperture 48 and with axis Aof the collar 24, and in turn, is in alignment with the receiveraperture 32 of the collar. Accordingly, in the preferred embodiment, theuser will be able to use the present invention to "point" with his orher index finger 170 to an area of the sample surface 12 that he or shewishes to take a colorimetric reading of the sample surface, and willthen simply press the switch 68 using the same index finger.

Referring back to FIGS. 4 and 5, the measured signals received by thesensor 10 and preprocessed by the circuitry 13 are transmitted back to ahost computer through a serial interface modular connector 58. Theserial interface connector 58 can be made to support the RS-232protocol, or can be configured to support the Apple Desktop Bus (ADB)protocol. Preferably, the serial interface connector 58 also suppliespower to the sensor circuitry, as provided by the host computer.

The reference photodetector and the sample photodetector 20,22 arepreferably identical devices and are preferably mounted back to back onthe printed circuit board so that, not only will the photodetectors besubstantially identical, they will share environmental characteristics,such as temperature, humidity, electrical noise, etc. Therefore, thephotodetectors 20,22 will be substantially thermally matched so thattemperature stabilization of the photodetectors is not required. Thisis, because the temperature variances will cancel each other out in thereflectance calculation as described below. The collar 24 also providesthermal isolation between the LEDs 18 and the sample photodetector 22,thus preventing heat generated by the LEDs from interfering with thethermal matching between the reference and sample photodetectors.Furthermore, no gratings or filters are required because the use of LEDsof different wavelengths eliminates the need for such gratings orfilters.

As shown in FIG. 6, each pin 60' of the reference photodetector 20 isoptionally coupled to a corresponding pin 60" of the samplephotodetector by a thermally conductive material 167. This material 167enhances the matching of thermal characteristics between the referencephotodetector 20 and the sample photodetector 22. If the correspondingpins 60', 60" are coupled to the same circuit connection, i.e., Vcc orground, the material 167 may also be electrically conductive so as tofacilitate the matching of electrical characteristics.

Those skilled in the art will appreciate that the sensor 10 need not bedesigned as mounted to a printed circuit board. A mounting plate or basecan be provided in place of the printed circuit board; or the collar 24or another component can be designed to house the LEDs andphotodetectors within the sensor 10 in the arrangement required by thepresent invention.

As shown in FIG. 7, circuitry for controlling the sensor 10 andpreprocessing the measured signals taken from the sensor comprises amicrocontroller 80, an analog to digital converter 82, a series of LEDswitches 84, and a programmable LED power source 86. The microcontroller80 is operatively coupled to the switch 68. The microcontrollercommunicates with the serial interface modular connector 58 through aserial communications bus 88. The microcontroller 80 can control whichof the LEDs 18 are activated through an LED select line 90 and cancontrol the intensity of the activated LEDs through an LED level line 92sent to the programmable LED power source 86. The programmable LED powersource provides the LED power through an LED power line 96, and alsoincludes an LED current sense input line 94 from a current detectorhoused within the sensor 10 as feedback to the programmable LED powersource calculation. However, since the sensor 10 is by nature adual-beam device, and emitter fluctuations are thereby cancelled duringthe process of measuring color, the LED current control circuit is notcritical to the operation of the device, but can be used to optimize LEDsignal to desired levels.

A sample channel output line 97 is coupled between a multiplexer device98 and the sample photodetector 22 housed within the sensor 10, and areference channel output line 99 is coupled between the multiplexerdevice 98 and the reference photodetector 20 housed within the sensor10. The microcontroller 80 is able to control which of the sample outletchannel line 97 or the reference channel output line 99 is sent to theanalog to digital converter device 82 through a multiplexer control line100, and the microcontroller 80 controls the operations of the analog todigital converter 82 through an A/D control line 102. The digitizedmeasurement from the sample photodetector 22 or the referencephotodetector 20 output by the analog to digital converter 82 is sentback to the microcontroller 80 via a digitized value line 104. As willbe apparent to one of ordinary skill in the art, the LED current senseline 94 can also be used to further correct the reference channelresponse for each individual LED against some expected response.

The color sensor 10 is operated as follows. To obtain reflectancereadings of a sample surface, the reflector cone aperture 48 is firstplaced on the sample surface 12 and the switch 68 is activated. Uponactivation of the switch 68, the microcontroller will take the digitized"dark" readings of the sample output channel 97 (Is_(d)) and of thereference output channel 99 (Ir_(d)) through the analog to digitalconverter device 82 (note that none of the LEDs 18 are activated). Next,the LED switches 84 are activated in sequence by the microcontroller 80,thus activating the LEDs 18 of the respective wavelengths in sequence.For each LED activated, the microcontroller takes the digitized readingsof the sample output channel 97 (Is.sub.λ) and of the reference outputchannel 99 (Ir.sub.λ), where "λ" represents the peak wavelength of theparticular LED activated. The raw, unscaled device reflectance Ru.sub.λfor each given LED of peak wavelength λ is then calculated using thefollowing equation: ##EQU1##

Generally, the microcontroller 80 measures Is_(d) and Ir_(d) right aftereach other, but just before the Is.sub.λ and Ir.sub.λ measurements aremade. This is an attempt to reduce the effect of photodetector offsetdrifts, which can be seen by the dark measurement. Similarly, Is.sub.λand Ir.sub.λ are measured right after each other for a given LED, whichis obviously an attempt to reduce photodetector gain drifts. Although itlogically follows that for each LED, a measurement of Is_(d) and Ir_(d)should be made immediately prior to or following the particular Is.sub.λand Ir.sub.λ measurement for that LED, a "shortcut" measurement cyclehas been developed and verified through experimentation that requiresonly one Is_(d) and Ir_(d) measurement to be taken for all LEDs duringthe measurement cycle. The shortcut measurement cycle is as follows:

    Measurement Cycle→Is.sub.d, Ir.sub.d, Is.sub.λ1, Ir.sub.λ1, Is.sub.λ2, Ir.sub.λ2 . . . Is.sub.λN, Ir.sub.λN

where N is the number of LEDs illuminated during the cycle.

Additionally, it has also been found that a slight gain in precision canbe obtained using the following measurement cycle:

    Measurement Cycle→Is.sub.d0, Ir.sub.d0, Is.sub.λ1, Ir.sub.λ1, Is.sub.λ2, Ir.sub.λ2 . . . Is.sub.λN, Ir.sub.λN, Is.sub.d1, Ir.sub.d1

From this the resulting value of Is_(d)λ and Ir_(d)λ for each given LEDof peak wavelength λ used in the dark correction scheme are obtained byinterpolating from the measured values Is_(d0), Ir_(d0), IS_(d1), andIr_(d1) as follows:

    Is.sub.dλ =((N Is.sub.d0 +Is.sub.d1)+(i-1)(Is.sub.d1 -Is.sub.d0))/(N+1)                                        (Equ. 2)

    for i=1 . . . N

and similarly

    Ir.sub.dλ =((N Ir.sub.d0 +Ir.sub.d1)+(i-1)(Ir.sub.d1 -Ir.sub.d0))/(N+1)                                        (Equ. 3)

where λi corresponds to measurements made for the ith LED in themeasurement cycle.

Preferably, at some point prior to measuring the reflectance of acolored sample surface, the color sensor should first be used to"calibrate" the photodetectors by activating the color sensor 10 over ablack or non-reflecting calibration surface to obtain a "black"reflectance reading R_(black), and then activating the color sensor 10over a reflecting diffuse white surface to obtain a "white" reflectancereading R_(white). The reflector cone aperture 48 is first placed on the"black" non-reflecting calibration surface, the switch 68 is pressed,and reflectance readings are made of the non-reflecting calibrationsurface. Then, the reflector cone aperture 48 is placed on the "white"reflecting calibration surface, the switch 68 is pressed, andreflectance readings are made of the reflecting calibration surface.These stored black and white calibration measurements R_(black) andR_(white) are then used to scale subsequent reflectance measurementsRu.sub.λ, described above, as follows:

    R.sub.λ =(Ru.sub.λ -R.sub.black)/(R.sub.white -R.sub.black))×(S.sub.w -S.sub.b)+S.sub.b           (Equ. 4)

where R.sub.λ is the normalized and bias corrected reflectancemeasurement for a given LED of peak wavelength λ; S_(w) is a "white"scaling factor obtained from a standard reference device, such as aGretag SPM100 laboratory grade spectrophotometer; and S_(b) is a "black"scaling factor obtained from a standard reference device. It alsofollows that:

    R'=((Ru'-R'.sub.black)/(R'.sub.white- R'.sub.black))×(S.sub.w -S.sub.b)+S.sub.b                                         (Equ. 5)

where R' is a vector of size N representing the normalized and biascorrected reflectance color value of the sample just measured; Ru' is avector of size N representing the raw, unscaled device reflectances ofthe sample just measured, R'_(black) is a vector of size N representingthe raw, unscaled device reflectances of a pre-measured "black"calibration tile; and R'_(white) is a vector of size N representing theraw, unscaled device reflectances of a pre-measured white calibrationtile.

In the above process, it is also within the scope of the invention toactivate more than one LED at a time, and to determine the reflectancefor the combined wavelengths of the illuminated LEDs. For example, toprovide better CIE color matching functions and to quicken the samplingprocess, a combination of LEDs can be illuminated to provide thecombined wavelengths for an entire RGB color wavelength band; thusrequiring only three samples to be taken and calculated--i.e., one forall LEDs making up the RED band, one for all LEDS making up the GREENband, and one for all LEDs making up the BLUE band. Additionally,multiple LEDs of the same wavelength may be activated simultaneously toincrease the apparent brightness associated with a particular wavelengthband.

As discussed above, it is within the scope of the invention to utilize alight-to-frequency converter device, such as a TSL230 from TexasInstruments, as a photodetector. The light-to-frequency converter deviceemits a frequency signal that corresponds to the brightness of the lightdetected by the device. Accordingly, use of a light-to-frequencyconverter device may eliminate the need for analog-to-digital converterdevices in the present invention, because the host computer ormicrocontroller will be able to calculate the reflectance based upon thefrequency of the signal transmitted by the light-to-frequency device.

An on-board programmable memory may optionally be included in thecircuitry to contain hardware setup and calibration information; andaccommodation of a liquid crystal display may also be included toprovide a user readout. Other enhancements can include an internalbattery for wireless operation.

The present embodiment for colorimetric operation utilizes ten LEDs ofdifferent wavelengths. Preferably, the peak wavelengths of the ten LEDsutilized are 430, 450, 470, 525, 558, 565, 585, 594, 610, and 635nanometers respectively. It is noted that although the peak wavelengthsare indicated, each LED transmits wavelengths of a particular bandwidth,which typically ranges from ten to one-hundred nanometers wide.

It should be apparent to one of ordinary skill in the art thatadditional or fewer LEDs may be utilized in the present invention,depending upon the accuracy and repeatability requirements of thesensor. For example, a sensor utilizing three or six LEDs will havelesser accuracy or repeatability than in the ten LED embodiment and asensor utilizing sixteen LEDs will have greater accuracy andrepeatability than the ten LED embodiment. Preferably, if a three LEDembodiment is utilized, the peak wavelengths of the three LEDs are 450,555, and 610 nanometers respectively; if a six LED embodiment isutilized, the peak wavelengths of the six LEDs are 450, 470, 512, 555,580, and 610 nanometers respectively; and if a sixteen LED embodiment isutilized, the peak wavelengths of the sixteen LEDs are 430, 450, 470,489, 512, 525, 558, 565, 574, 585, 594, 605, 610, 620, 635, and 660nanometers respectively.

Although it is within the scope of the invention to utilize the threeLED embodiment (i.e., 450, 555, and 610 nanometers respectively), thereare several practical advantages for using more than three spectrallyunique LEDs or channels for collecting data. First, using more thanthree spectral shapes makes it easier to span the space defined by theCIE color matching functions, since each additional channel provides anadditional degree of freedom with which to compute tristimulus values.

Second, in practice, it is difficult to cover the space defined by thecolor matching functions for one particular illuminant with acombination of only three LEDs. This is due to the fact that there is alimited number of LED spectral shapes commercially and physicallyavailable, and the LED power spectral distributions vary significantlybetween LEDs from a single lot. Therefore, by using more than three LEDsthe instrument will not be as sensitive to known variations in the LEDs(due to redundancy). In addition, combining LEDs will provide effectiveshapes that are different than those provided by single LEDs. This canfacilitate in matching the multi-peak CIE color matching functions.

Finally, using more than three channels will allow the device to providecolorimetric information for the sample under measure for multipleilluminants without obtaining the entire spectral reflectance of thesample. This capability results in an instrument with significantflexibility over a three channel device.

Once the corrected, reflectance ratio vector R' of the sample isdetermined as described above, this ratio vector R' is then convertedinto a usable color measurement value t (for example, the colormeasurement value t can be the CIE XYZ tristimulus value for the sample)through a linear or a non-linear operation. To execute this conversion,a mathematical profile of the sensor 10, which obtained the measurement,must first be determined. This is because the conversion from thereflectance ratio vector R' to the color measurement value t dependsupon the unique spectral characteristics of the sensor components usedto obtain the reflectance ratio vector R'.

The mathematical profile of the sensor 10 is based upon the followingmatrix/vector equation:

    R'=S.sup.T Dr+b                                            (Equ. 6)

where (assuming M is the number of LEDs 18 and N is the number ofwavelengths to be sampled in the visible spectrum) R' is an M×1 vectorof the recorded reflectance ratios; S^(T) is an M×N matrix representingthe spectral power distributions of the M spectrally unique LEDs 18; Dis an N×N diagonal matrix representing the spectral sensitivity of thesample photodetector; b is an M×1 bias vector; and r is an N×1 vectorrepresenting the actual spectral reflectance of the sample. Themathematical profile of the device is determined by finding S^(T), D,and b. These values are found by taking measurements of samples havingknown spectral reflectances r, using the sensor 10. In simple terms, thedetermination of the mathematical sensor profile is based on thespectral sensitivity of the sample photodetector, the spectral powerdistributions of the LEDs 18, and the reflectance of several knownsamples.

The spectral sensitivity D of the sample photodetector is available fromthe photodetector's manufacturer. Provided a quality photodetector isused, the sensitivity should be the same, within an acceptable range,over all detectors. At the time of the present invention, LEDmanufacturers are not able to ensure the exact spectral shape and peakwavelength across a particular batch of LEDs. For this reason, it isnecessary to measure the spectral power distribution S^(T) of each LEDwith a spectraradiometer. But because, the spectraradiometer providesonly the relative shape of the distribution, the absolute power of theLEDs and the bias vector b are determined by performing a measurementwith the sensor 10 on each of two spectrally known samples having knownreflectances r₁ and r₂. Typically, black and white samples are used.These measurements provide M sets of 2 equations to solve for the twounknowns (S^(T) and b), where we already have each row of S^(T) towithin a constant:

    R.sub.1 '=S.sup.T Dr.sub.1 +b                              (Equ. 7a)

    R.sub.2 '=S.sup.T Dr.sub.2 +b                              (Equ. 7b)

Once the sensor is profiled, i.e., once S^(T), D and b are found, anaccurate estimate of the actual spectral reflectance r of the sample canbe obtained. The CIE XYZ tristimulus value t for the sample underilluminant L can be denoted by the vector/matrix equation:

    t=A.sup.T Lr                                               (Equ. 8)

where L is an N×N diagonal matrix containing the illuminant spectralpower distribution and the columns of the N×3 matrix A contain the CIEXYZ color matching functions. The determination or approximation of taccording to Equ. 8 will be apparent to one of ordinary skill in theart. Mathematically the problem of transforming from the ratios to adescriptor like t can be described as: ##EQU2## where E{} is theexpectation operation over the system noise and reflectance spectra ofinterest, F is a function which transforms from the color spacecontaining t to a perceptually uniform color space (i.e., accounts forthe sensitivity of the human visual system), and G is the functionapproximating t (G(c)≈t). Accordingly, G is the function to be found.Depending upon the application, it may be desirable to select G aseither a linear or non-linear function.

For applications in which a spectrally widely varying set of samples ismeasured, a linear transformation will be more robust than non-lineartransformations. Typically, the function F will be of a form which makesan analytical solution to the above optimization problem difficult. Inthis case, the transformation G can be found by a two step process. Inthe first step, an initial estimate for G is obtained analytically whichminimizes the error in the CIE XYZ space (i.e., the function F isignored). This solution is easily computed using simple matrix algebra.In the second step, this estimate is used as a starting point for anumerical optimization algorithm which minimizes the above non-linearproblem. Any standard non-linear optimization algorithm will besufficient for this task.

The expectation operator in the above optimization problem is taken overa set (or ensemble) of reflectance spectra. The above approach requiressome representative reflectance samples which the sensor may be used tomeasure. From these samples, a reflectance correlation matrix isconstructed and used in the first analytical step (when the function Fis ignored). Specifically, the analytical solution is given by:

    G=A.sup.T K.sub.r S S.sup.T K.sub.r S!.sup.-1              (Equ. 10)

where K_(r) =E{rr^(T) } is the reflectance correlation matrix which isestimated by: ##EQU3## where R is some ensemble of N_(R) reflectancespectra. The numerical step is then performed over each sample in R. Theconditioning of the transformation matrix can be set as an optimizationconstraint. The amount of conditioning or regularization should be afunction of the system noise thereby producing a transformation whichgives good repeatability.

Alternatively the function F can be locally linearized for each samplein the ensemble R. This linearization provides a means to obtainanalytically a solution which may be perceptually acceptable, and allowthe incorporation of the system noise for good repeatability.

Finally, for the spectral case, the optimization problem becomes:##EQU4## for which the analytical solution is given by:

    G=K.sub.r S S.sup.T K.sub.r S!.sup.-1                      (Equ. 13)

Depending upon the system noise and the LEDs it may be necessary toperform a pseudo-inverse operation for computing the above inverse. Thepseudo-inverse operation may also be needed in the colorimetric case.The pseudo-inverse can be computed by dropping numerically insignificantsingular values in the matrix S^(T) K_(r) S.

As shown in FIG. 14, the present invention, by installing aphotodetector 160 in place of an LED, can also be used as a gloss meter.The photodetector 160 would have to replace a first LED which ispositioned 180° away from a second LED 18" with respect to axis A. Thephotodetector would thus be able to detect the specular component of thelight waves transmitted from the second LED and reflected off thesample.

It should also be apparent to one of ordinary skill in the art that LEDs18 having wavelengths in the non-visible spectrum can be utilized forvarious purposes. For example, utilizing LEDs in the infrared spectrumwill allow the sensor to measure the infrared reflectivity of a samplesurface. In a related application, the infrared LEDs can also be used totransmit data to a host computer through the reflector cone's aperture48; thus eliminating the need for the serial interface connector 58.

The present invention can also be used to measure the radiance of agiven sample that is self luminous, i.e., a CRT display screen. Thisapplication does not need to utilize the LEDs.

As shown in FIG. 8, an alternate circuit for the present inventionutilizes a successive approximation technique to perform measurement ofspectral reflectances from the dual beam sensor 10. The circuitry forperforming the successive approximation measurement comprises amicrocontroller 100, a digital to analog converter 102, a D latch 104,another D latch 106, a sample channel voltage comparator device 108, areference channel voltage comparator device 110, and an OR gate 112. Themicrocontroller is operatively coupled to the switch 68. Themicrocontroller 100 communicates with the serial interface modularconnector 58 through a communications bus 114. The microcontroller cancontrol which of the LEDs 18 are activated through an LED select line116, which indicates which of the LED switches 118 are to be activatedor inactivated. The approximation circuit also utilizes a voltagereference 120, which is a fixed stable voltage reference used by thedigital to analog converter 102 and the reference channel voltagecomparator device 110. The approximation circuit also utilizes an LEDvariable control device 122, which provides variable resistance used togate the amount of current allowed to flow from the LED's current drain.This device is fed by an LED gate signal 124 sent from a delay rampcircuit 126 that generates a voltage ramp beginning at a low voltagelevel, and over time, becomes higher in a linear fashion.

The approximation circuit is initialized as follows: Initially, uponactivated by the switch 68, the microcontroller activates a clear line128 which clears both D latches 104,106; the microcontroller transmitsto the digital to analog converter 102 a 0.5 full scale code through aD/A control line 130; and the internal LED select variable i (whichindicates the particular LED switch to be activated) is initialized tozero (no LED selected).

The reflectivity measurement process for a given LED i is conducted asfollows: The microcontroller 100 will first select LED i through the LEDselect line 116 sent to the LED switches 118. LED power 132 is appliedto the LED i, but the LIED i does not yet activate because the LEDvariable control device 122 is "off." Next, the microcontroller 100activates a "start" line 134, which is coupled to the clock input 136 ofthe D latch 106. Accordingly, the Q output 138 of the D latch 106 ("LEDon") becomes activated as the "start" signal ripples through the D latch106. This "LED on" signal 138 is sent to the delay ramp circuit 126,which in turn begins applying the linearly increasing "LED gate" signal124 to the LED variable control device 122. The LED variable controldevice 122, now activated, allows current to flow from the LED powersource 132 through the previously selected LED switch 118 through thecorresponding LED 18 and LED cathode 140 and into an "LED current drain"142. Of course, since current is flowing through the LED 18, the LED nowemits light at its predetermined wavelength; however, the brightnessdepends upon the linearly increasing "LED gate" signal 124.

The reference photodetector 20, as discussed above, receives light fromthe activated LED 18 and transmits a signal indicative of the measuredlight to the voltage comparator 108 over a reference channel output line144. Likewise, the sample photodetector 22 receives light reflected fromthe color sample, as discussed above, and transmits a signal indicativeof the detection of this light to the voltage comparator circuit 108over a sample channel output line 146. As the LED gate signal 124increases, intensity of light from the LED 18 increases until either thesample output voltage comparator 108 or the reference output voltagecomparator 110 is activated. The sample channel voltage comparator 106will be activated if the voltage on the sample channel output line 146is greater than the analog output 152 from the digital to analogcomparator 102, and the reference channel voltage comparator 104 will beactivated if the reference channel output line 144 voltage is largerthan the voltage reference 120.

If the sample output voltage comparator 108 is activated, it activatesthe "X>Y" line 148, which is coupled to the clock input 149 of the Dlatch 104. Therefore, upon the "X>Y" 148 signal being activated by thesample channel voltage comparator, the D latch 104 will correspondinglynotify the microcontroller 100 over the "X>Y (held)" line 154. At thistime, the microcontroller 100 will determine if the granularity(resolution) of the D/A setting has been reached and, if so, thismeasurement is reported to the host computer through the RS232 seriallink 58, and the measurement is complete. If the microcontroller 100determines that the granularity of D/A setting has not been reached, thedigital to analog signal is increased over the digital to analog controlline 130, the microcontroller sets the "clear line" 128 clearing both Dlatches 104,106 and restarts the above measurement process beginningwith activating the "start" line 134.

Alternatively, if the reference channel voltage comparator 110 isactivated, this indicates that the reference channel output 144 voltageis larger than the voltage reference 120. Activation of the referencechannel comparator 108 thus causes the D latch 106 to be cleared. Themicrocontroller 100 will sense this, through a "fault" signal, and willdecrease the D/A control signal 130 to the digital to analog converter102, and will again proceed with the above measurement process startingby activating the "start line" 134.

The above successive approximation circuit can be used to measure thesample channel output 146 with respect to the reference channel output144. This method uses the digital to analog (D/A) converter 102 to set athreshold level 152 against which the sample channel output 146 iscompared. If the threshold level is too low, it is increased; if it istoo high, it is decreased. The threshold value being sought is thatwhich matches the sample channel output 146 at the closest possibleinstant to when the reference channel output 144 matches the stablevoltage reference 120. This method of "hunting" for the appropriatethreshold is iterative, and when certain optimizations, such as a binarytree search algorithm, are employed, it can be relatively quick. Whenthe final threshold is discerned, the digital representationcorresponding to the sample channel is known. Since the stable voltagereference 120 is both used to generate the output voltage 152 of the D/A102, as well as in the comparison of the reference photocliode channeloutput 144, the act of comparing the sample photodiode channel output146 to the D/A output voltage 152 is effectively the same as the directcomparison of the sample photodiode channel output 146 with thereference photodiode channel. Furthermore, since the output voltage 152of the D/A 102 is defined by the following relationship:

    D/A output=voltage reference×(digital setting/full scale digital value)

and since at the moment when the comparison is valid, the referencephotodiode channel output 144 is nearly equivalent to the voltagereference 120 while the sample photodiode channel output 146 is nearlyequivalent to the D/A output, the above relation becomes:

    (sample chan./ref. chan.)=(digital setting/full scale digital value)

Therefore, the act of finding the digital setting corresponding to theappropriate threshold is the same as finding the value of sample channeldivided by the reference channel.

Of course, the above circuit is equally valid when a means to scale thesample and/or reference channels by known quantities is used. Inaddition, assuming linearity of the output of the combination of the"delay (ramp)" 126 and "LED variable control" 122 circuits, the binarytree search algorithm can even be further improved by noting the timediscrepancy between the triggering of the sample channel comparison andthe triggering of the reference channel comparison, and scaling nextresulting "expected" threshold setting accordingly. Finally, it is alsovalid that the reference channel output 144 be used as the referencevoltage by which the D/A output voltage 152 is generated; this is themost direct means of calculating the desired value of sample channeloutput divided by reference channel output.

As shown in FIGS. 12a-12c, an alternate embodiment of the annular collar24' has a plurality of emitter apertures 72 angled at a 45° angletowards the axis A, eliminating the need for the 22.5° conicalreflective surface within a reflector cone, and eliminates the need forthe reflector cone altogether. It is noted that the different sizesdepicted for the emitter apertures 72 corresponds to different sizes ofLEDs used.

As shown in FIG. 13, an alternate embodiment of the invention includesan optical lens 76 in place of the reflective surfaces of the reflectorcone. The optical lens acts to bend the lightwaves emitted from theemitter apertures 30 to a 45° angle towards the axis A.

It is also within the scope of certain aspects of the present inventionto provide a "low-end" color sensor that does not utilize the referencephotodetector 20, yet may utilize any or all of the novel elementsdescribed above for use with a hand-held, single-beam color sensor.Referring to FIG. 7, an important element in such a single-beam colorsensor would be the LED current sense input line 94. It would also bebeneficial in such an embodiment to inject the current sense signalsinto the sample photodetector 22 and an associated amplification circuit(whether internal or external to the sample photodetector) to provideadvanced gain and offset correction.

As mentioned above, alternate embodiments of the invention as shown inFIGS. 16 and 17 may be better suited for remote color sensing of asample surface, such as a patient's skin. As shown in FIG. 16, areflector cone 28" includes a conical reflector surface 52" that isangled with respect to the axes B of the emitter apertures 30" at a 10°angle. Light emitted through the emitter apertures 30" and reflectedfrom the conical reflective surface 52" will contact the sample surface12 at an angle of 20°. Therefore the embodiment as shown in FIG. 16 willhave a 20°/0° geometry. As shown in FIG. 17, a collar 24'" includes aradially outwardly extending, and substantially conical reflectivesurface 190, which intersects axes B such that light emitted through theemitter apertures 30'" will be reflected towards a curved, innerreflective surface 192 of a reflector cone 28'". The light reflectedfrom the inner reflective surface 192 towards the sample surface 12 willthus be substantially diffuse. Therefore, the embodiment as shown inFIG. 17 will have a modified-diffuse/0° geometry.

Having described the invention in detail and by reference to preferredembodiments thereof, it will be apparent that modifications andvariations are possible without departing from the scope of theinvention defined in the appended claims.

What is claimed is:
 1. A hand-manipulatable device for gatheringreflective, densitometric, spectrophotometric, colorimetric,self-luminous or radiometric readings from a sample surface,comprising:a housing having a substantially flat bottom surface and atop surface contoured to fit comfortably in the fingers and palm of thehuman hand; a sensor mounted to said housing, including a focal apertureand including circuitry and optics for performing a reflective,densitometric, spectrophotometric or colorimetric reading from a portionof said sample surface aligned with said focal aperture; a switchoperatively coupled to said sensor for activating said sensor to performsaid reading; and a data link adapted to be operatively coupled betweensaid sensor and a computer for relaying data from said sensor to saidcomputer; said sensor including (a) a printed circuit board mountedwithin said housing, (b) a plurality of light sources, mounted withinsaid housing, respectively emitting light of a substantially differentwavelength band, (c) a sample photodetector mounted to said printedcircuit board, (d) a first optical element to direct a first portion oflight emitted by each of said light sources to the sample surface, and(e) a second optical element to direct a portion of light reflected fromthe sample to said sample photodetector.
 2. The hand-manipulatabledevice of claim 1, wherein said focal aperture is in substantialvertical alignment with an area in said top surface of said housing forseating a tip of an index finger.
 3. The hand-manipulatable device ofclaim 2, wherein said switch is a pressure activated switch mounted tosaid housing substantially in said area for seating a tip of an indexfinger.
 4. The hand manipulatable device of claim 1, wherein:saidprinted circuit board includes a plurality of apertures extendingcompletely therethrough; at least one aperture is provided for acorresponding one of said light sources; and each of said sources arepositioned substantially adjacent to said corresponding aperture.
 5. Thehand manipulatable device of claim 4, wherein said light sources arelight emitting diodes which are mounted to a daughterboard, thedaughterboard being coupled to said printed circuit board.
 6. Ahand-manipulatable device for gathering reflective, densitometric,spectrophotometric, colorimetric, self-luminous or radiometric readingsfrom a sample surface, comprising:a housing having a substantially flatbottom surface and a top surface contoured to fit comfortably in thefingers and palm of the human hand; a sensor mounted to said housing,including a focal aperture and including circuitry and optics forperforming a reflective, densitometric, spectrophotometric orcolorimetric reading from a portion of said sample surface aligned withsaid focal aperture; a switch operatively coupled to said sensor foractivating said sensor to perform said reading; and a data link adaptedto be operatively coupled between said sensor and a computer forrelaying data from said sensor to said computer; said sensor including(a) a printed circuit board mounted within said housing, (b) a pluralityof light sources, mounted within said housing, respectively emittinglight of a substantially different wavelength band, (c) a samplephotodetector mounted to said printed circuit board, (d) a first opticalelement to direct a first portion of light emitted by each of said lightsources to the sample surface, (e) a second optical element to direct aportion of light reflected from the sample to said sample photodetector,(f) a reference photodetector mounted to said printed circuit board, and(g) a third optical element to direct a second portion of light emittedby each of said light sources to said reference photodetector.
 7. Thehand manipulatable device of claim 6, wherein said third optical elementis an optical cap having a substantially non-absorbing interiorintegrating surface, mounted over said apertures and said referencephotodetector, said optical cap forming a reference chamber, whereby,said second portion of light emitted by said light sources reflects offof said interior integrating surface of said optical cap and saidreference photodetector receives a substantial amount of said secondportion of light reflected off of said interior integrating surface. 8.The hand manipulatable device of claim 7, wherein said optical cap isintegral with said housing.
 9. The hand manipulatable device of claim 6,wherein said reference photodetector and said sample photodetector aremounted substantially back-to-back so as to share environmentalcharacteristics.
 10. A hand-manipulatable device for gatheringreflective, densitometric, spectrophotometric, colorimetric,self-luminous or radiometric readings from a sample surface,comprising:a housing having a substantially flat bottom surface and atop surface contoured to fit comfortably in the fingers and palm of thehuman hand; a sensor mounted to said housing, including a focal apertureand including circuitry and optics for performing a reflective,densitometric, spectrophotometric or colorimetric reading from a portionof said sample surface aligned with said focal aperture; a switchoperatively coupled to said sensor for activating said sensor to performsaid reading; and a data link adapted to be operatively coupled betweensaid sensor and a computer for relaying data from said sensor to saidcomputer; said sensor including (a) a printed circuit board mountedwithin said housing, (b) a plurality of light sources, mounted withinsaid housing, respectively emitting light of a substantially differentwavelength band, (c) a sample photodetector mounted to said printedcircuit board, (d) a first optical element to direct a first portion oflight emitted by each of said light sources to the sample surface, and(e) a second optical element to direct a portion of light reflected fromthe sample to said sample photodetector; said printed circuit boardincluding a plurality of apertures extending completely therethrough, atleast one aperture being provided for a corresponding one of said lightsources, and each of said sources being positioned substantiallyadjacent to said corresponding aperture; said light sources beingarranged about said sample photodetector; said second optical elementbeing mounted over said sample photodetector and including a receivingaperture in optical alignment with said sample photodetector; said firstoptical element including a reflector cone, said reflector coneincluding hollow cavity, a tip, and said focal aperture in said tip,said focal aperture providing optical communication into said hollowcavity; said reflector cone being mounted over said apertures and saidsecond optical element to form a sample chamber, said focal aperturebeing positioned in optical alignment with said receiving aperture; saidreflector cone including a frustoconically shaped reflective innersurface, axially aligned with said focal aperture, and positioned tointersect with said first portion of light emitted by said lightsources; and said reflective inner surface of said hollow cavity beingangled inwardly toward said focal aperture at an angle so as to directsaid first portion of light emitted by said light emitting toward sampleat substantially a 45° angle, such that said first portion of lightreflects from the sample at substantially a 45° angle and said samplephotodetector receives a diffuse component of said light reflected fromthe sample.
 11. A hand-manipulatable device for gathering reflective,densitometric, spectrophotometric, colorimetric, self-luminous orradiometric readings from a sample surface, comprising:a housing havinga substantially flat bottom surface and a top surface contoured to fitcomfortably in the fingers and palm of the human hand; a sensor mountedto said housing, including a focal aperture and including circuitry andoptics for performing a reflective, densitometric, spectrophotometric orcolorimetric reading from a portion of said sample surface aligned withsaid focal aperture; a switch operatively coupled to said sensor foractivating said sensor to perform said reading; and a data link adaptedto be operatively coupled between said sensor and a computer forrelaying data from said sensor to said computer; said sensor including(a) a plurality of light emitting diodes, each of said diodesrespectively emitting light of a substantially different wavelengthband, mounted within said housing, (b) a light-pipe for directing lightemitted from said diodes to said optics, said optics being adapted totransmit light directed from said light pipe to said focal aperture atan angle of approximately 45°, (c) a reference photodetector forreceiving light bled from said light-pipe, and (d) a samplephotodetector for receiving a diffuse component of said lighttransmitted to said focal aperture by said optics and reflected from asample surface.
 12. A hand-manipulatable device for gatheringreflective, densitometric, spectrophotometric, colorimetric,self-luminous or radiometric readings from a sample surface,comprising:a housing having a substantially flat bottom surface and atop surface contoured to fit comfortably in the fingers and palm of thehuman hand; a sensor mounted to said housing, including a focal apertureand including circuitry and optics for performing a reflective,densitometric, spectrophotometric or colorimetric reading from a portionof said sample surface aligned with said focal aperture; a switchoperatively coupled to said sensor for activating said sensor to performsaid reading; and a data link adapted to be operatively coupled betweensaid sensor and a computer for relaying data from said sensor to saidcomputer; said sensor including (a) a plurality of light sources,mounted within said housing, respectively emitting light of asubstantially different wavelength band, (b) a sample photodetectormounted within said housing, (c) a first optical element to direct afirst portion of light emitted by each of said light sources to thesample surface, and (d) a second optical element to direct a portion oflight reflected from the sample to said sample photodetector.
 13. Thehand-manipulatable device of claim 12, wherein said sensor furtherincludes (e) a reference photodetector mounted within said housing, and(f) a third optical element adapted to direct a second portion of lightemitted by each of said light sources to said reference photodetector.14. The hand-manipulatable device of claim 13, wherein said samplephotodetector and said reference photodetector are mounted in closeproximity to one another so as to share environmental characteristics.15. The hand-manipulatable device of claim 12, wherein said focalaperture is in substantial vertical alignment with an area in said topsurface of said housing for seating a tip of an index finger.
 16. Ahand-manipulatable device for gathering reflective, densitometric,spectrophotometric, colorimetric, self-luminous or radiometric readingsfrom a sample surface, comprising:a housing having a substantially flatbottom surface and a top surface contoured to fit comfortably in thefingers and palm of the human hand; a sensor mounted to said housing,including a focal aperture and including circuitry and optics forperforming a reflective, densitometric, spectrophotometric orcolorimetric reading from a portion of said sample surface aligned withsaid focal aperture; a switch operatively coupled to said sensor foractivating said sensor to perform said reading; and a data link adaptedto be operatively coupled between said sensor and a computer forrelaying data from said sensor to said computer; said sensor including(a) a plurality of light sources, mounted within said housing,respectively emitting light of a substantially different wavelengthband, (b) a sample photodetector mounted within said housing, (c) meansfor directing a first portion of light emitted by each of said lightsources to the sample surface, and (d) means for directing a portion oflight reflected from the sample to said sample photodetector.